Mobile devices equipped with high-resolution cameras and displays are capable of capturing large volumes of multimedia content that can be stored locally on the mobile device. However, transferring and sharing of such content real-time among devices and servers require short-range connectivity modems capable of delivering speeds of 20 gigabits per second (Gbps) or higher. The dilemma for the mobile industry is that the capability of radio technologies for wireless modems such as Wireless Local Area Network (WLAN) are not keeping up with the exponential growth of multimedia content capabilities of mobile devices.
Without compression of the multimedia content, wired connectivity via cables is the only way to transfer the large amount of data among devices or between a device and the cloud. Currently the wired cable High-Definition Multimedia Interface (HDMI) 2.0b specification provides up to 18 Gbps of bandwidth and will support 4K Ultra High Definition (UHD) at 3480×2160 pixels per frame and 60 frames per second (fps) of video content. The upcoming HDMI 2.1 specification will provide up to 48 Gbps of bandwidth and will support 8K UHD at 7680×4320 pixels and 60 fps of video content. The HDMI 2.1 specification will also provide support for 4K UHD at 120 fps.
High throughput WLAN IEEE standards, such as 802.11ac and 802.11ad, have been introduced to increase WLAN throughput. However, these standards are still no adequate to meet the throughput demands of uncompressed 4K and 8 k UHD. IEEE 802.11ac has a maximum theoretical transfer rate 1.3 Gbps. IEEE 802.11ac Wave 2 has a maximum theoretical transfer rate 2.34 Gbps. IEEE 802.11ad has a maximum theoretical transfer rate of 7 Gbps. These current standards will not support uncompressed 4K and 8K UHD video transfer. Lossy compression such as h.264 or h.265 is typically used to reduce the needed bit rate.
The data throughput of radio transceivers is inherently limited by the availability of commercial radio spectrum and their limited tuning range. For example, in IEEE802.11ac standard, the channel bandwidths are specified at either 80 MHz or 160 MHz in the 5 GHz ISM band. With channel bonding, WLAN 802.11ac transceivers have been operated at speeds of 866 Mbps. A single user operating at such speeds will use up the entire bandwidth of the local network! The next-generation of 802.11ac utilizes Mu-MIMO feature (Multi-user, Multi-Input, Multi-Output) using smart antenna technology to focus dedicated radio beams on individual users thereby increasing total bandwidth through spatial diversity. Mu-MIMO is expected to deliver up to 1.7 Gbps using a 4×4 MIMO configuration.
The IEEE802.11ad standard (WiGig™) takes advantage of higher spectrum availability in the 57-64 GHz band (V-band). With about 7 GHz spectrum available, speeds of 5 Gbps have been demonstrated and potential speeds of 7 Gbps using multi-carrier OFDM modulation are being pursued. Millimeter-wave radio transceivers have significant complexity and cost associated therewith and suffer from multi-path and other channel impairments in an indoor operation. Additionally, due to excessive atmospheric absorption at V-band, the transmission range of WLAN 802.11ad modems is limited between 2 and 3 meters even with complex antenna beam forming feature. While 802.11ad modems are being introduced to market, the industry is continuously looking for higher bandwidth wireless modems for next-generation devices.
Transmission in the optical region of the spectrum offers distinct advantages over radio waves for wireless communications. The much higher carrier frequency in the optical range enables much higher modulation bandwidth. Atmospheric absorption in the optical region between infrared (IR) and ultraviolet (UV) can be much lower than that in the mm-wave and Terahertz (THz) frequencies depending on the choice of optical wavelengths. This improves the communication range of optical wireless modem and reduces power consumption of the transmitter. Available transmit source power from microwave and mm-wave solid-state devices declines with increasing carrier frequency as 1/f2. Even with recent developments in GaN power transistor technology, achieving power level of 1 W at the Terahertz frequencies in high volume is not practical. Wireless 802.11ad modems are designed to operate at a range of 2 to 3 meters with 10 mW transmitted power level in the V-band using CMOS and SiGe BiCMOS power amplifiers. In the future, even if additional radio spectrum is made available at higher frequency bands (e.g. Terahertz bands), radio modems face increasing atmospheric attenuation and lower available transmit power. In contrast, due to recent developments in GaN and GaAs based LED and laser technologies, optical power levels have been steadily increasing. CREE has demonstrated GaN LEDs with 300 lumens optical output power per watt of dissipated power. Single-die LED arrays with 18,000 lumens are commercially available today. Unlike radio communication, optical wireless links do not require licenses to operate in the visible or nearly-visible spectrum enabling OEMs to get products faster to market.
While lasers can be used as optical sources for the transceiver module disclosed, lasers require precise line-of-sight alignment with the detector, which is not practical for mobile devices in typical consumer use-cases. Defocusing of lasers can increase projection angle for the transmitter, but it requires expensive optics, packaging and assembly processes. LEDs, on the other hand, can operate over a wide projection angles and can be operated in a point-to-multipoint network configuration (e.g. wireless optical network in a room).
Multi-color LEDs can be employed at relatively low cost to create wavelength diversity in the optical link, thereby multiplying the data throughput of the link. Disclosed herein are multi-wavelength LEDs to demonstrate full-duplex (simultaneous uplink and downlink) optical communication. Multi-wavelength LED transmitters also offer the option of wavelength modulation which provides more immunity to ambient interferences and a more robust link. TABLE 1 summarizes the commercially available LED technologies, their associated wavelength, built-in voltage and the semiconductor materials on which they are fabricated.
TABLE 1WavelengthVoltageSemiconductorColor[nm][ΔV]materialInfraredλ > 760ΔV < 1.63GaAs, AlGaAsRed610 < λ < 7601.63 < ΔV < 2.03AlGaAs, GaAsP,AlGaInP, GaPOrange590 < λ < 6102.03 < ΔV < 2.10GaAsP, AlGaInP, GaPYellow570 < λ < 5902.10 < ΔV < 2.18GaAsP, AlGaInP, GaPGreen500 < λ < 5701.9 < ΔV < 4.0GaP, AlGaInP, AlGaP,InGaN/GaNBlue450 < λ < 5002.48 < ΔV < 3.7 ZnSe, InGaN on SiCsubstrateViolet400 < λ < 4502.76 < ΔV < 4.0 InGaN on SiC substrateUltravioletλ < 4003.1 < ΔV < 4.4Diamond (235 nm),AlGaN, AlN (210 nm)WhiteBroadΔV = 3.5 Blue/UV diode withspectrumyellow phosphorous
Another important design factor for achieving high speed optical transmitters and receivers is how individual LEDs and detectors are scaled to achieve the desired signal levels. Bright LEDs are manufactured today in the form of arrays of smaller LEDs on a single for thermal management reasons. A typical LED chip can consist of 8×8 or 16×16 LED arrays. In today's LED bulbs, these arrays of micro-LEDs are powered together to achieve the desired brightness. The individual micro-LEDs in the array can be independently driven as individual transmitters to form MIMO (multi-input, multi-output) spatial diversity for the link, which further increase data throughput. MIMO is often used in radio wireless transceivers to deal with multi-path interference and fading in the channel. However, MIMO technique requires much higher complexity at the transmitter and receiver (CCD or CMOS photo sensor) thereby considerably increasing the cost of the system.
The micro-LEDs in today's LED arrays are designed for static Direct Current (DC) operation and are not suitable for high-frequency operation. Typical dimensions of a micro-LED and detectors may be on the order of 50-100 μm on a side. At these dimensions, the resistive capacitive (RC) time constant of the device is too high and uniform AC operation of the entire diode active area cannot be achieved due to spreading resistance of the anode and cathode contacts.
The substrates of choice for most high-volume LEDs are either GaAs or SiC. Both material technologies lend themselves well for microwave and ultra high-speed circuit implementations due to their semi-insulating characteristics. In fact, GaAs and SiC/GaN materials today form the backbone of microwave and millimeter-wave IC's for commercial and military applications. Global Communications Semiconductor (GCS) Incorporated offers commercially available GaAs and SiC/GaN based LED processes for product implementations.
Ultra High-Speed, Short-Range Connectivity for Mobile Devices
According to International Data Corporation (IDC), Worldwide Quarterly Mobile Phone Tracker, worldwide smart phone shipments was estimated at 1.2 B units in 2014 and expected to reach 1.8 B units in 2018, resulting in a 12.3% compounded annual growth rate (CAGR) from 2013-2018. IDC also reports 245M units of tablet shipments in 2014. The majority of these phones and tablets will be equipped with high-resolution displays and cameras and can be potential candidates for ultra-high-speed, wireless optical links. In addition to smart phone and tablets, other mobile devices such as high-resolution digital cameras and camcorders, notebook computers and portable disk drives currently using USB (Universal Serial Bus) as well as fixed devices such digital TV's, projectors. Additional development of wireless optical dongles capable of replacing USB3 data and HDMI2.0 (4K) video cables may be provided. Wireless optical modules for embedded applications in a wide range of electronic devices such as but not limited to a smart watch, a smart phone, a tablet, a laptop, a digital camera, a digital camcorder, a computer monitor, a TV, a projector, and a wireless access point may be provided. The overall addressable market size for ultra-wideband wireless optical transceivers can be well in excess of 2 B units by 2018. The adoption and attach rate of wireless optical transceivers to mobile devices should follow similar trend as WLAN modems integration into mobile devices. Based on preliminary analysis, the cost of the wireless, optical transceiver will be less than 802.11ad mm-Wave radio modems while its performance will be significantly higher.
Indoor Wireless Networking for Home and Enterprise Applications
There is growing interest in using LEDs for in-building wireless networking and location-based services. The LiFi™ Consortium was established November 2011 to enable Giga-speed visible light communication (VLC) applications. The Infrared Data Association (IrDA) (www.irda.org) also announced in 2011 working groups for the standardization of 5 Gbps and 10 Gbps IrDA using IR sources while such products have not reached the market yet.
There is an active worldwide effort to develop standards for VLC. The IEEE802.15.7 WPAN (Wireless Personal Area Network) Task Group 7 has completed MAC and PHY layers specifications for VLC. The Japan Electronics and Information Technology Industries Association has released the JEITA CP-1221 standard for VLC systems. While the standardization activities have focused on the physical layer and application layer of VLC systems, the fundamental speed limitations of VLC transceivers remains the main, stumbling block to the realization of practical, ultra-speed VLC transceiver.
There is also growing interest in using VLC for indoor enterprise applications. Philips has announced the Shopping Assistant application using LED-based communication with mobile devices. In this application, the consumer can download information about each product as they browse the store.
Smart Lighting for Automotive Transportation
Another potential market for LED-based VLCs is the automotive transportation market. LED lighting due to their superior brightness and power consumption is becoming the technology of choice for automobile headlights, taillights, streetlights and traffic signaling. Smart LED array technology offers auto manufacturers the ability to incorporate new functionality in the external and internal lighting of next-generation automobiles. VLC offers auto manufacturers potential of auto-to-auto, and auto-to-streetlight and auto-to-traffic light communications. This generates enormous potential for new applications in auto transportation. For example, auto-to-street headlight VLC can enable in-car wireless hot spots that can be used to access the Internet. Also, traffic lights can warn incoming cars about cross-section traffic and improve safety. Cars facing obstacles can warn several cars behind them to slow down and avoid potential accidents.
Another arena for WOC is the application of ultra-high-speed LED transceivers for 100 Gbps Ethernet backplanes for data center applications. With the enormous growth of cloud-based services and computing driven by social multi-media, the cost and power consumption of 100 Gbpes Ethernet connections is becoming a major concern. Today's Ethernet cards employ copper connectivity using 10 Gbase-T transceivers, which incur significant power consumption to drive copper cables at 10 Gbps over distances of several meters. Ultra-high-speed LED transceivers combined with low-cost multi-mode fibers can significantly reduce the power consumption of Ethernet links in this application.
There are other applications of optical wireless applications such as secure communications for mobile financial transactions but there is growing adoption of Near Field Communications (NFC) technology in mobile devices. Ultra-high-speed, wireless optical communication has military applications as well. For examples, reconnaissance Unmanned Air Systems (UAS) (or drones) acquire large amounts of imagery data and sensor information, which are downloaded directly to ground or through satellite via radio links. The radio links have bandwidth limitations and subject to interference and eavesdropping. Optical wireless links can offer a more secure air-to-ground, ultra-high-speed link to down load UAS information to ground terminals. There has been demonstrated 1 Gbps video stream downlink from an aircraft moving at an altitude of 7 km and speed of 800 km/h. Lasers were used as the optical source with complex alignment mechanics for the laser and the ground-based terminal. The solutions disclosed herein can eliminate the need for complex line-of-sight alignment systems that are required for laser free-space optical systems. While this system is designed for specific military application, it has potential applications to the commercial UAS market and demonstrates the viability of ultra-wideband optical, wireless communication over large distances.
While the potential market for LED-based ultra-high-speed wireless communication is substantial, innovative approaches are required to develop and build ultra-high-speed LED transceivers. The “white” LED device design and optimization have been driven by the industry to achieve the lowest cost and power consumption for a target level of brightness. No considerations have been given to the switching speed of LEDs for communication applications. While attempts have been made to use off-the-shelf LEDs for wireless optical communications, such attempts have had little success due to intrinsic speed limitation of the micro-LEDs and detectors.